In order that the instant invention may be clearly understood, it is useful to review the prior art relating to photochemical isotope separation. U.S. Pat. No. 2,713,025 and British Pat. No. 1,237,474 are good examples of processes for the photochemical separation of the isotopes of mercury. The first requirement for a photochemical isotope separation is that one finds conditions such that atoms or molecules of one isotope of a given element absorb light more strongly than do atoms or molecules of another isotope of said element. Mercury is a volatile metal and readily forms a vapor of atoms. Said atoms absorb ultraviolet light at 2537 A. The absorption line of Hg.sup.202 is displaced by about 0.01 A with respect to the absorption line of Hg.sup.200. Since absorption lines are extremely narrow, one may by use of a light in a critically narrow wavelength region excite either Hg.sup.200 or Hg.sup.202 without substantially exciting the other, depending on the exact wavelength used.
The second requirement for a photochemical isotope separation is that those atoms or molecules which are excited by light undergo some process which the atoms or molecules which have not been excited to not undergo, or at least do not undergo as rapidly. A quantum of 2537 A ultraviolet light imparts an excitation of 112.7 Kcal/mole to the mercury atom which absorbs it. The number of mercury atoms which at room temperature are thermally excited to this energy is vanishingly small, hence the atoms excited by light are not diluted by atoms excited by thermal means. Atoms of this high excitation readily undergo reactions with H.sub.2 O (as taught in the U.S. patent) or with O.sub.2, HCl or butadiene (as taught in the British patent), said reactions not occurring at room temperature with unexcited mercury.
Uranium, however, is a highly refractory metal, boiling only at extremely high temperatures. Thus, use of the above-described process with uranium atoms instead of mercury involves obvious difficulties. The most volatile form of uranium is UF.sub.6. U.sup.235 F.sub.6 and U.sup.238 F.sub.6 both absorb ultraviolet light and do so to exactly the same extent at all wavelengths in the UV; hence, UV excitation of UF.sub.6 does not satisfy the first requirement of photochemical isotope separation. However, UF.sub.6 will absorb infrared light in the region around 626 cm.sup..sup.-1 (the V.sub.3 band) and 189 cm.sup..sup.-1 (the V.sub.4 band), and at the various other wave lengths, in the IR region, reported by McDowell et al., Journal of Chemical Physics, 61, pp. 3571-3580. Both the V.sub.3 and V.sub.4 bands of U.sup.235 F.sub.6 are shifted slightly toward higher energy with respect to the V.sub.3 and V bands of U.sup.238 F.sub.6 respectively, but the size of these shifts is small compared to the width of the bands; in other words, the infrared absorption spectra of U.sup.238 F.sub.6 and U.sup.235 F.sub.6 do not exactly coincide, but they overlap at all wavelengths so that if one isotope absorbs light, so, to a substantial degree, will the other. Hence the infrared excitation of UF.sub.6 by absorption of single IR photon is a process of limited isotopic selectivity.
The second requirement for isotope separation is also a matter of some difficulty for UF.sub.6. UF.sub.6 molecules which are excited by IR light are no different from molecules which have received the same energy by thermal excitation. Whatever process the photo-excited molecules will undergo, those molecules which are thermally excited to the same energy will also undergo. This dilution of the photoexcited molecules with thermally excited molecules will further decrease the isotopic separation factor.